Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between two other layers. The active layer emits light when holes and electrons from the outer layers combine in the active layer. The holes and electrons are provided by passing a current through the LED. In one common configuration, the LED is powered through an electrode that overlies the top layer and a contact that provides an electrical connection to the bottom layer.
The cost of LEDs is an important factor in determining the rate at which this new technology will replace conventional light sources and be utilized in high-power applications. The cost of the LEDs is, in part, determined by the yield of the LEDs from the wafers on which they are constructed. In general, the wafer includes a large number of LEDs with each LED being separated from its neighboring LEDs by a dicing street. When the LEDs are separated from the wafer, cuts are made in the dicing street area thereby releasing individual dies. The size of the dicing streets is typically 100 μm. This area is basically wasted space. If the LEDs are large compared to the dicing streets, the overall percentage loss introduced by the dicing streets is relatively small, and hence, acceptable. Unfortunately, the ratio of the street dimensions to the LED dies is significant in many LED applications. For example, the losses inherent in a 1 mm die introduced by the dicing streets results in a 20 percent loss of area on the wafer. In many applications, dies that are as small as a half a millimeter are required. In these cases the losses are even worse.
Accordingly, it would be advantageous to provide a dicing scheme in which the dicing streets are smaller. Unfortunately dicing schemes that depend on mechanical cutting or laser scribing are limited to dicing streets of the order of 50 μm. Furthermore schemes that depend on etching the underlying wafer are also limited by the thickness of the wafer, since the aspect ratio of the width of a trench to the depth of the trench is limiting.
In addition, the time required to dice a wafer having a large number of small dies on the wafer is significant. Since the processing time increases the cost of the dies, a dicing scheme in which all of the dies are released at once would be advantageous.